1. Field of the Invention
The present invention relates to a semiconductor circuit and a resistance value controlling method. In particular, the invention relates to a semiconductor circuit having a resistor circuit including a transistor, and a resistance value controlling method used for the semiconductor circuit.
2. Description of Related Art
In recent years, an operation speed of a semiconductor integrated circuit has been increased. As the operational speed of the integrated circuit increases, an influence of a reflection signal as a reflected transmission signal becomes large. In order to suppress the reflection signal, a terminal resistor is provided to an input buffer circuit for impedance matching. Accordingly, it is desirable to use a high-accuracy terminal resistor.
However, a resistance value of the terminal resistor varies depending on use conditions or the like. For example, the resistance value of the terminal resistor varies in accordance with change in ambient temperature or applied power supply voltage, for example. Further, a resistance value fluctuates due to manufacturing process variations. If the resistance value of the terminal resistor is shifted from a predetermined value, impedance mismatching occurs.
To that end, a semiconductor circuit for correcting a resistance value of the terminal resistor is disclosed. For example, in an interface circuit as disclosed in Japanese Unexamined Patent Application Publication No. 2000-307410, control signals S1 and S2 from the terminal resistor control circuit are input to the terminal resistor composed of a transistor. Then, the control signals S1 and S2 are changed based on an external input signal INb to thereby control the resistance value of the terminal resistor. Further, a circuit as disclosed in Japanese Unexamined Patent Application Publication No. 2-170570 also outputs a control signal based on an external input signal to control a resistance value of the terminal resistor. Circuits as disclosed in Japanese Unexamined Patent Application Publication No. 2003-122465 also control a resistance value of the terminal resistor using a control signal from a correction circuit.
However, the above semiconductor circuits have a problem in that an external input signal is necessary for generating a control signal for controlling the terminal resistor. Further, in the circuit as disclosed in Japanese Unexamined Patent Application Publication No. 2003-122465, although an external input signal is unnecessary, a correction circuit and a detection circuit composed of plural resistive elements and transistors are necessary. This leads to overheads of the circuit and size.